Color-plus-clear coating systems involving the application of a colored or pigmented base coat to a substrate followed by the application of a transparent or clear topcoat to the base coat have become very popular as original finishes for automobiles. The color-plus-clear systems have outstanding gloss and distinctness of image. The clear coat is particularly important for these properties.
Because of almost constant exposure to ultraviolet radiation (sunlight) in automotive applications, it is imperative that coating compositions used in such applications be resistant to degradation caused by ultraviolet radiation exposure. Degradation of coating systems due to ultraviolet light exposure can cause appearance problems, and in severe cases, coating adhesion failure and delamination of one or more coating layers.
Prior art attempts to improve the ultraviolet resistance of coating compositions have included the addition of expensive organic UV(A) absorbers and hindered amine light stabilizers to compositions including primers, base coats, and clear coats. These organic additives are themselves susceptible to photodegradation, limiting their effectiveness with time. These small organic molecules are mobile within cured and uncured coating systems and as a result can migrate, for example, from a clear coating into underlying coated layers, reducing the protection provided to the upper layers. Furthermore, these materials are often volatile and may be partially lost from coatings that are cured at elevated temperatures. Other efforts involve addition of opaque pigments such as carbon black, but such pigments are not suitable for use in transparent (clear coat) compositions.
U.S. Pat. No. 5,846,310 to Noguchi et al. discloses silica particles coated with particles of TiO2, Fe2O3, ZnO or ZrO2, for use as pigments in coatings, or as sunscreening agents. The biphasic nature of these particles, however, can lead to problems of haze and poor durability. The refractive indices of TiO2, Fe2O3, ZnO and ZrO2 are significantly higher than that of a plastic or coating composition, which rapidly leads to haze with increasing phase size and concentration. In addition, TiO2 and, to a lesser extent Fe2O3, are photoactive which can result in accelerated photodegradation of the coating or plastic which is typically undesirable. Also, ZnO is chemically unstable to acids and bases. Furthermore, Fe2O3 can lead to coloration and ZrO2 does not absorb UV(A).